Research efforts have been made to develop a recombinant yeast capable of expressing a heterologous protein, e.g., hepatitis B surface antigen (HBsAg), in the secretion pathway, thereby simplifying the purification of the yeast-derived recombinant protein.
The gene product of PEP4, protease A, regulates several yeast vacuolar hydrolases at the post-translational level. Woolford et al., Mol. Cell Biol. 6: 2500-2510, 1986. In earlier studies, HBsAg expressed in the secretion pathway was found to be toxic to various protease A-deficient strains of host yeast. See, e.g., Jones, Genetics 85: 23-33, 1977; Zubenco et al., Genetics 102: 679-690, 1982; and Achstetter et al., Yeast 1: 139-157, 1985. According to a more recent report, such toxicity could be progressively reduced in media containing lower concentrations of ammonium sulphate; the non-inhibitory transformants thus obtained were characterized by the phenotypes of enlarged cell and colony morphology, dimorphic transition to pseudohyphal-like and invasive growth in nitrogen-starved solid media, increase in HBsAg particle production, and enhancement of growth rate in liquid media. Chen et al., Curr Genet 27: 201-206, 1995.